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and other vertebrates as was at one time assumed. But in
order to explain this very important part of the subject it will
be necessary to give some account of the development of the
teeth. The type selected is the Hedgehog, which has been
recently and carefully described by Dr. Leche of Stockholm,

Fro. S7.—Two stages in th© development of the teeth of a Mammal (diagrammatic sec-
tions), (dv, Bone of alveolus j dent, dentine; dent.s, dental sac ; <w, enamel ;
tsn.m, enamel membrane ; <m.m2, enamel membrane of permanent tooth ; en.ptpt
enamel pulp ; grt dental groove; lam, dental lamina ; lamf, part of dental lamina
which grows downwards below the tooth germ j n, neck connecting germs of milk
and permanent tooth ; pwp* dental papilla ; .potjp3, dental papilla of permanent tooth.
(After O.-Hertwig.)
which type has furthermore the advantage of being a " central"
type of mammal. The first step In the formation of the teeth
is a continuous invagination of the epithelium covering the jaw
to form, a deepish wall of tissue running in the thickness of the
jaw ; this is perfectly continuous from end to end of the lower
jaw. From this " common enamel germ" (Schmelzleiste of the
Germans1) "special enamel germs " (Schmelzorgane, enamel organs)
are developed here and there as thickenings in the form of buds
1 JforpA. Jo&r&, xix. 1892i p. 502.